Switching regulator sweep starting protection circuit

ABSTRACT

An overload protection system for a switching power supply includes a variable rate and variable pulse duration drive source for operating the switching transistor. Upon turn-on of the power supply, the switching transistor is driven with low repetition rate pulses having a predetermined time duration. The rate is gradually increased to a normal operating rate to minimize turnon transients. Load sensing circuitry is used in conjunction with the variable rate drive source to cause the drive source to operate at the low rate in the event that an overload is applied to the power supply, and to turn off the drive source if the overload persists. Circuitry for providing automatic reset following an overload is employed. A feedback circuit is used to vary the duration of the drive pulses to provide automatic voltage regulation.

Hawkins SWITCHING REGULATOR SWEEP STARTING PROTECTION CIRCUIT [75]Inventor: George C. Hawkins, Hanover Park, Ill.

[73] Assignee: Motorola, Inc., Franklin Park, Ill.

[22] Filed: Feb. 3, 1972 [21] Appl. No.: 223,099

[52] US Cl. ..32l/47, 321/18, 323/22 T, 321/45 S [51] Int. Cl. ..H02m7/52 [58] Field of Search ..321/45 R, 45 S; 323/22 T [56] ReferencesCited UNITED STATES PATENTS 3,376,478 4/l968 Sheng et al. ..3l7/3l3,458,796 7/1969 Cassady ..321/45 S 3,506,907 4/1970 Porterfield et a1......32l/45 R 3,629,622 12/1971 Denenberg, Jr ..323/22 T 11] 3,733,540[451 May 15, 1973 A tt0rney Vincent J. Rauner et a].

[5 7] ABSTRACT An overload protection system for a switching powersupply includes a variable rate and variable pulse duration drive sourcefor operating the switching transistor. Upon turn-on of the powersupply, the switching transistor is driven with low repetition ratepulses having a predetermined time duration. .The rate is graduallyincreased to a normal operating rate to minimize turn-on transients.Load sensing circuitry is used in conjunction with the variable ratedrive source to cause the drive source to operate at the low rate in theevent that an overload is applied to the power supply, and to turn offthe drive source if the overload persists. Circuitry for providingautomatic reset following an overload is employed. A feedback circuit isused to vary the duration of the drive pulses to provide automaticvoltage regulation.

10 Claims, 2 Drawing Figures PATENIEDMY 1 51m 3' 733,540

SHEET 1 OF 2 REGULATED DC OUTPUT W l 3 lm V 22 1 34 TO AC 2 SOURCE 1: T

DRIVER L 25 55 30 STARTlNG VARIABLE PULSE PULSE WIDTH WIDTH 1' CONTROLMONOSTABLE c1 RCU IT VARIABLE FREQUENCY FREQUENCY CONTROL OSCILLATORCIRCUIT T OVERLOAD SENSING a CIRCUIT PATEHTED MY 1 973 SHEET 2 0F 2wum30m BACKGROUND This invention relates generally to power supplies,and more particularly to power supplies employing a switching circuitfor regulating the output voltage of the supply.

There are many applications wherein it is desired to provide aself-protected, self-resetting power supply. One such system utilizingsuch a power supply is a communications radio transmitter.

Several protected, self-resetting power supplies are known. One suchsystem employs circuitry for turning off the switching oscillator in theevent of an overload and for turning on the oscillator a predeterminedtime following the overload. In another such system, the oscillator isturned on at a reduced amplitude following the overload, and theamplitude of oscillations is gradually increased to reduce turn-ontransients.

Whereas these techniques provide power supplies that are suitable tocertain applications, the first technique is not readily applicable toheavily filtered power supplies because of transients generated in thefilter during turn-on. The second technique requires the use ofswitching transistors capable of withstanding the power dissipatedduring turn-on when the switching transistors are operated in theirlinear mode.

SUMMARY It is an object of the present invention to provide a switchingpower supply having automatic overload protection and automatic reset.

It is a further object of this invention to provide a switching powersupply that can be operated into a highly reactive load without damageto its components.

It is another object of this invention to provide a switching powersupply having a controlled voltage build-up.

In accordance with a preferred embodiment of the invention, a variablepulse width multivibrator is employed to drive a switching oftransistor. A variable oscillator is used to drive the multivibrator.The pulse width of the multivibrator signal is varied to adjust theoutput voltage of the power supply. The frequency of the oscillator isreduced during turn-on and during the presence of an overload. Followingturn-on, the frequency of the oscillator is gradually increased, therebyproviding a controlled voltage build-up at the output of the powersupply. Reducing the oscillator frequency during an overload limits theamount of current delivered to the overload, thereby protecting thepower supply. In the event that the overload persists, the oscillator isturned off for a predetermined time duration, and is periodically turnedon at the reduced frequency until the overload is removed, at which timethe frequency is gradually increased until normal operation is achieved.

DESCRIPTION OF THE DRAWINGS cuit according to the invention.

DETAILED DESCRIPTION Referring to FIG. 1, there is shown a block diagramof the power supply according to the invention. A transformer 5 isconnected to a source of alternating current voltage and to arectifier-filter circuit comprising rectifier bridge and capacitor 12. Aswitch means such as series transistor switch is connected to the outputof rectifier 10 and to the input of a filter circuit comprising diode21, filter choke 22 and capacitor 23. Switch 15 may be driven by anysuitable activating means compatible with the characteristics of switch15, including such as, for example, driver transistors, oscillators andmonostables. Output voltage from the power supply is provided at anoutput point 24. A driver stage 25 is connected to transistor switch 15.A pulse width control circuit is connected to output point 24 and to avariable pulse width monostable 35, which is in turn connected to driver25. Pulse width control circuit 30, monostable and driver 25 comprisepulse control means for switch 15 in this embodiment. A frequencycontrol circuit is also connected to output point 24 and to a variablefrequency oscillator v45, which is connected to variable pulse widthmonostable 35. Frequency control circuit 40 and oscillator comprise ratecontrol means for switch 15 in this embodiment. An OR gate is connectedfor controlling monostable 35 and oscillator 45. A starting circuit 55,which includes timing means, is connected between the output of bridge10 and an input of OR gate 50. Similarly, an overload sensing circuit 60is connected between output point 24 and the other input of OR gate 50.Starting circuit 55, OR gate 50 and sensing circuit 60 serve asprotection means for the circuit of this embodiment.

In operation, switch 15 is driven by driver 25 to provide current pulsesto filter 20. The variable pulse width monostable 35 and the pulse widthcontrol circuit 30 determine the time duration of the current pulses,while the variable frequency oscillator 45, which drives the variablepulse width monostable 35, and the frequency control circuit 40determine the repetition rate of the current pulses. The startingcircuit causes the OR gate 50 to enable monostable 35 and oscillator 45for a predetermined time duration following turn-on of the power supply.Overload sensing circuit causes monostable 35 and oscillator 45 toremain operative following turn-on when the power supply is operatingnormally.

When the power supply is turned on, starting circuit 55 causes OR gate50 to apply power to, or otherwise enable, monostable 35 and oscillator45 for a predetermined time duration, in this embodiment, 200milliseconds. Oscillator 45 begins to drive monostable 35 at a rateconsiderably lower than the normal operating frequency of the powersupply. In this embodiment, the starting frequency is approximately 1KHz, and the operating frequency is approximately 20 KHz. The monostable35 provides pulses having a maximum pulse width of 40 microseconds todriver 25 for driving transistor switch 15 and switch 15 providescurrent pulses having a time duration of 40 microseconds and a l KHzrepetition rate to filter 20. Current begins to flow through choke 22into capacitor 23 as a result of the current pulses from switch 15. Thecurrent thus initiated continues to flow through choke 22, capacitor 23and diode 21 for a predetermined time following the current pulse fromswitch 15. The time that current continues to flow is determined by thetime constant of choke 22 and capacitor 23. In this embodiment, the timebetween current pulses during power supply turnon is chosen to begreater than the amount of time required for current to stop flowingthrough choke 22 in order to reduce the effects of the startingtransients of filter on switch 15. The current flow through filter choke22 and capacitor 23 causes a voltage to build up across capacitor 23.This voltage is applied to frequency control circuit 40 to increase theoperating frequency of oscillator 45 as the output voltage acrosscapacitor 23 increases. As the frequency of oscillator 45 increases, thevoltage at output point 24 will increase until its value reaches apredetermined operating level. This is normally achieved within thepredetermined time set by starting circuit 55. After the voltage atoutput point 24 has reached its predetermined value, the overloadsensing circuit 60 provides a signal to OR gate 50 to maintainmonostable 35 and oscillator 45 operative. Pulse width control circuitsenses the voltage at output point 24 and adjusts the width of theoutput pulses from monostable to maintain the voltage at output point 24constant.

Should an overload, such as, for example, a short circuit be connectedto output point 24 during turn-on, the voltage at output point 24 willnot increase. The frequency of variable frequency oscillator will bemaintained at its low rate to limit the number of current pulses passingthrough switch 15, thereby maintaining the average current throughswitch 15 at a value which prevents damage to the supply. If theoverload persists, overload sensing circuit 60 will not provide an on.signal to OR gate 50, and gate will disable monostable 35 and oscillator45 after the starting signal from starting circuit is removed. The powersupply will remain disabled for a predetermined time period, in thisembodiment 1 second, after which time the normal starting procedure willbe repeated. If the overload has been removed, the power supply willreturn to normal operation. If not, the supply will cycle between its onand off states until the overload is removed. If the overload is appliedafter the supply has been turned on, the frequency of oscillator 45 willbe immediately reduced to its lowest rate, and will remain at the lowrate until the overload is removed or until shut-off by starting circuit55, after which time the supply will operate in a fashion similar to itsoperation when it is started into an overload. If the supply has beenoperating longer than the starting time determined by starting circuit55, shut-off will occur immediately after the supply is overloaded.

Referring to FIG. 2, there is shown a schematic diagram ofa power supplyaccording to the invention. The functional blocks of the diagram of FIG.1 are defined by dashed lines encircling the circuitry providing thefunction. The blocks are identified by like numbers in each figure.Transistor switch 15 comprises a transistor 16 having a collectorconnected to rectifier-filter l0 and an emitter connected to filter 20.Driver 25 comprises transistors 26, 27 and 28 and associated components.Transistors 26, 27 and 28 are connected toform a conventional amplifierwell known in the art. The input to the amplifier is the base oftransistor26, which is connected to multivibrator 35, and the output ofthe amplifier is the collector of transistor 28, which is connected tothe base of transistor 16.

Variable pulse width monostable 35 comprises a standard monostablemultivibrator including transistors 36 and 37, and utilizes an emitterfollower transistor 38 and a diode 39 to provide a low impedancecharging path for timing capacitor 29. The output of multivibrator 35 isthe collector of transistor 36, which is connectedto the input of driver25 at the base of transistor 26 Pulse width control circuit 30 comprisesa transistor 33 having a base which is coupled to output point 24through diode l7, variable resistor 32 and zener diode 31. A transistor34 having a collector connected to the collector of transistor 37 and abase connected to the emitter of transistor 33 comprises the outputstage of pulse width control circuit 30.

Variable frequency oscillator 45 is a conventional astable multivibratorcomprising transistors 47 and 48, and includes emitter follower outputscomprising transistors 71 and 72 for driving timing capacitors 73 and74, respectively. A variable current source comprising transistor havinga base connected to frequency control circuit 40 and a collectorconnected to diodes 76 and 77 supplies current to timing capacitors 73and 74 through diodes 76 and 77, respectively. Resistors 78 and 79 areconnected between the power supply bus and capacitors 73 and 74,respectively, and supply current to capacitors 73 and 74.

Frequency control circuit 40 comprises a transistor 41 having a baseconnected to output point 24 and resistors 42 and 43 which areconnnected between the collector of transistor 41 and the base oftransistor 75 of oscillator 45. A capacitor 44 is connected between hejunction of resistors 42 and 43 and the power supply bus for oscillator45.

The OR gate 50, which enables monostable 35 and oscillator 45, comprisesa diode 51 having an anode connected to the anode of a zener diode 52,and a capacitor 53 which is connected in parallel with zener diode 52.

The overload sensing circuit 60 comprises a transistor 61 having acollector connected to the cathode of diode 51 of OR gate 50 and anemitter connected through a resistor 88 to output point 24 of the powersupply. A base of transistor 61 is connected through a resistor 89 to azener diode 62 which serves as a voltage reference. A pair of biasingdiodes 63 and 64 are connected in series between the base of transistor61 and output 24.

The starting circuit 55 comprises an astable multivibrator includingtransistors 56 and 57. Starting circuit 55 also includes a multivibratorcontrol transistor 58 and amplifier transistors 81, 82 and 83 which forman amplifier coupling the collector of multivibrator transistor 57 to aninput of OR gate 50.

When the power supply is initially turned on, transistor 57 provides apositive output pulse to transistor 81 to turn on transistors 81, 82 and83, thereby completing a circuit between the output of rectifier bridge10 and zener diode 52. Zener diode 52 breaks down, causing a regulatedvoltage to be applied to the power supply bus that supplies power tomonostable 35 and oscillator 45. Oscillator 45 begins to oscillate at arate determined by the time constants of resistor 78 and capacitor 73',and resistor 79 and capacitor 74. This is the low and capacitor 91 whichis connected to the collector of transistor 48. The differentiatedpulses are applied to the base of transistor 37 of monostable 35 througha diode 93. The differentiated pulses applied to the base of transistor37 cause transistor 37 to turn on and transistor 36 to turn off, therebycausing the voltage appearing at the collector of transistor 36 toincrease. The voltage at the collector of transistor 36 remains highuntil capacitor 29 is charged through resistor 19, diode 38 andtransistor 37 to a sufficient level to allow transistor 36 to turn on.The time required to charge capacitor 29 determines the duration of thepositive pulse applied to driver 25.

The positive pulse from the collector of transistor 36 is applied to thebase of transistor 26 of driver 25 and is amplified thereby. The pulseis subsequently applied to the transistors 27 and 28 for furtheramplification. The amplified pulse appears at the collector oftransistor 28 and is applied to the base of transistor 16 of switch 15,thereby rendering transistor 16 conductive during the duration of thepositive pulse.

When transistor 16 becomes conductive, current flows from rectifierbridge through transistor 15, filter choke 20 and capacitor 23, causinga voltage to appear at output point 24. The voltage appearing at outputpoint-24 is applied to the base of transistor 41, thereby renderingtransistor 41 conductive and reducing the voltage at the collector oftransistor 41. When transistor 41 is made conductive, current flows fromthe power supply bus through, capacitor 44, resistor 42 and transistor41. The voltage at the junction of capacitor 44 and resistor 42 drops ata rate determined by the charging time of capacitor 44. As the voltageat the junction of capacitor 44 and resistor 42 drops, the base oftransistor 75, which is coupled to the aforesaid junction throughresistor 43, is forward biased, thereby rendering transistor 75conductive..The degree of conductivity of transistor 75 is determined bythe voltage at the junction of capacitor 44 and resistor 42. Astransistor 75 becomes conductive, charging current is supplied bytransistor 75 to capacitors 73 and 74 through diodes 76 and 77,respectively, thereby reducing the charging time of capacitors 73 and74, and increasing the frequency of oscillator 45. Under normaloperating conditions, oscillator 45 reaches its maximum oscillationfrequency within a length of time less than the duration of the pulseprovidedby starting circuit 55 to OR gate 50.

As the frequency of oscillation of oscillator 45 approaches its maximumoperating value, the voltage at output point 24 approaches its desiredvalue. The output voltage is applied to zener diode 62 of the overloadsensing circuit 60 through resistor 89, the series connected diodes 63and 64, and the series combination of resistor 88 and the base-emitterjunction of transistor 61. When the valueof the voltage at output point24 is sufficiently high to cause zener diode 62 to break down, currentflows through the base emitter-junction of transistor 61, renderingtransistor 61 conductive. This completes a circuit between output point24 and the anode of diode 51 of OR gate 50, thereby enabling OR gate 50to continue supplying power to monostable 35 and oscillator 45 after thestarting pulse from starting circuit 55 has passed. Transistor 58, whichis also coupled to the collector of transistor 61 is also renderedconductive thereby resetting the starting circuit multivibratorcomprising transistors 56 and 57 and removing the starting pulse from ORgate 50.

Regulation of the output voltage of the power supply is obtained asfollows. As the voltage at output point 24 approaches its regulatedvalue, zener diode 31 of pulse width control circuit 30 breaks downallowing current to flow through zener diode 31, resistor 32, and thebase-emitter junctions of transistors 33 and 34, thereby makingtransistors 33 and 34 conductive. The collector of transistor 34 isconnected to the collector of transistor 37 and limits the level towhich the voltage at the collector of transistor 37 can rise whentransistor 37 is turned off. This in turn limits the charge that can beplace on capacitor 29 by emitter follower 38 when transistor 37 is offand transistor 36 is on. As the voltage at output point 24 increases,transistor 34 becomes more conductive, further limiting the charge oncapacitor 29. As the charge placed on capacitor 29 by emitter follower38 is reduced, the amount of time required to charge capacitor 29through resistor 19 when transistor 37 is rendered conductive decreases,and the width of the pulse supplied by monostable 35 is correspondinglydecreased. This in turn reduces the amount of time that transistor 16remains conductive and prevents the voltage at output point 24 fromexceeding a predetermined value. The value of the voltage at outputpoint 24 may be adjusted by means of variable resistor 32.

In the event that an overload is connected to output point 24, thevoltage at point 24 will drop, and render transistor 61 nonconductive.The voltage applied to the anode of diode 51 of OR gate 50 will drop tosubstantially zero, causing oscillator 45 and monostable 35 to bedisabled, thereby disabling the entire power supply. Simultaneously, thestarting circuit 55 will be enabled and will provide starting pulses toOR gate 50, and the power supply will cycle between its low frequencyswitching state and its off state until the overload is removed.

In summary, the techniques of the instant invention provide a way toachieve controlled start-up and overload protection for a switchingpower supply without the use of dissipative linear elements. Thevariable frequency starting and protection circuitry is fully compatiblewith pulse width modulation voltage regulation circuitry to provide afully regulated protected high power supply.

I claim:

1. A switching power supply control circuit for controlling the outputvoltage applied to an output point from a direct current source,including in combination, switch means connected to said source and tosaid output point for controlling the output voltage delivered to saidoutput point by said source, said switch means having a conductive statefor passing current to said load and a nonconductive blocking state,pulse control means for rendering said switch means conductive for apredetermined time duration connected to said switch means, said pulsecontrol means being further connected to said output point andresponsive to the voltage thereat for adjusting said time duration in amanner tending to maintain said output voltage substantially constant,rate control means for causing said switch means to be renderedalternately conductive and nonconductive at a predetermined rateconnected to said pulse control means, said rate control means beingfurther connected to said output point and responsive to the voltagethereat for causing said switch means to be alternately renderedconductive and nonconductive at a rate lower than said predeterminedrate when the output voltage at said output point is below apredetermined level.

2. A control circuit as recited in claim 1 further including protectionmeans coupled to said pulse control means and to said output point, saidprotection means being responsive to said output voltage for maintainingsaid pulse control means operative to render said switch meansconductive when said output voltage exceeds a predetermined value, andstarting means coupled to said pulse control means for rendering saidpulse control means operative for a predetermined time interval, saidcontrol means thereby being rendered operative by said starting meansand maintained operative after said predetermined time interval by saidprotection means when the output voltage exceeds a predetermined level.

3. A switching power supply control circuit as recited in claim 1wherein said pulse control means includes monostable means for applyingpulses to said switch means connected to said switch means, saidmonostable means being further connected to said output point andresponsive to the voltage thereat for altering the width of said pulsesto increase the time duration thereof for increasing the time durationsaid switch is rendered conductive when said output voltage drops belowa predetermined level.

4. A switching power supply control system as recited in claim 3 whereinsaid rate control means includes voltage controlled oscillator meansconnected to said monostable means for providing oscillations thereto,said oscillator means being connected to said output point andresponsive to the voltage thereat for varying the frequency of saidoscillations, said frequency being decreased in response to a decreasein said output voltage.

5. A starting and protection circuit for a power supply utilizing anelectronic switch having conductive and nonconductive states forcontrolling the output voltage applied to an output point of saidsupply, including in combination, means for periodically activating saidswitch to the conductive state at a predetermined periodic rateconnected to said switch, means for controlling said rate connected tosaid activating means and to said output point, said rate control meansbeing responsive to the voltage at said output point for decreasing saidperiodic rate in response to a decrease in said out put voltage and forincreasing said periodic rate in response to an increase in said outputvoltage.

6. A starting and protection circuit as recited in claim 5 furtherincluding sensing means coupled to said activating means and to saidoutput point for maintaining said activating means operative when saidoutput voltage exceeds a predetermined level.

7. A starting and protection circuit as recited in claim 6 furtherincluding starting means coupled to said activating means for renderingsaid activating means operative for a predetermined time interval.

8. A starting and protection circuit as recited in claim 7 furtherincluding OR circuit means having inputs connected to said startingmeans and said sensing means, and an output connected to said activatingmeans, said OR circuit means being responsive to said starting means forrendering said activating means operative for a predetermined timeinterval, and to said sensing means for maintaining said activatingmeans operative after said predetermined time interval when said outputvoltage exceeds a predetermined level.

9. A starting and protection circuit as recited in claim 8 wherein saidstarting means includes means for repetitively rendering said activatingmeans operative.

10. A starting and protection circuit as recited in claim 8 furtherincluding control means connected to said activating means and to saidoutput point, said control means being responsive to the voltage at saidoutput point for increasing the time during each period that said switchis maintained conductive in response to a decrease in the output voltageand for decreasing the time during each period that said switch ismaintained conductive in response to an increase in the output voltage.

1. A switching power supply control circuit for controlling the outputvoltage applied to an output point from a direct current source,including in combination, switch means connected to said source and tosaid output point for controlling the output voltage delivered to saidoutput point by said source, said switch means having a conductive statefor passing current to said load and a nonconductive blocking state,pulse control means for rendering said switch means conductive for apredetermined time duration connected to said switch means, said pulsecontrol means being further connected to said output point andresponsive to the voltage thereat for adjusting said time duration in amanner tending to maintain said output voltage substantially constant,rate control means for causing said switch means to be renderedalternately conductive and nonconductive at a predetermined rateconnected to said pulse control means, said rate control means beingfurther connected to said output point and responsive to the voltagethereat for causing said switch means to be alternately renderedconductive and nonconductive at a rate lower than said predeterminedrate when the output voltage at said output point is below apredetermined level.
 2. A control circuit as recited in claim 1 furtherincluding protection means coupled to said pulse control means and tosaid output point, said protection means being responsive to said outputvoltage for maintaining said pulse control means operative to rendersaid switch means conductive when said output voltage exceeds apredetermined value, and starting means coupled to said pulse controlmeans for rendering said pulse control means operative for apredetermined time interval, said control means thereby being renderedoperative by said starting means and maintained operative after saidpredetermined time interval by said protection means when the outputvoltage exceeds a predetermined level.
 3. A switching power supplycontrol circuit as recited in claim 1 wherein said pulse control meansincludes monostable means for applying pulses to said switch meansconnected to said switch means, said monostable means being furtherconnected to said output point and responsive to the voltage thereat foraltering the width of said pulses to increase the time duration thereoffor increasing the time duration said switch is rendered conductive whensaid output voltage drops below a predetermined level.
 4. A switchingpower supply control system as recited in claim 3 wherein said ratecontrol means includes voltage controlled oscillator means connected tosaid monostable means for providing oscillations thereto, saidoscillator means being connected to said output point and responsive tothe voltage thereat for varying the frequency of said oscillations, saidfrequency being decreased in response to a decrease in said outputvoltage.
 5. A starting and protection circuit for a power supplyutilizing an electronic switch having conductive and nonconductIvestates for controlling the output voltage applied to an output point ofsaid supply, including in combination, means for periodically activatingsaid switch to the conductive state at a predetermined periodic rateconnected to said switch, means for controlling said rate connected tosaid activating means and to said output point, said rate control meansbeing responsive to the voltage at said output point for decreasing saidperiodic rate in response to a decrease in said output voltage and forincreasing said periodic rate in response to an increase in said outputvoltage.
 6. A starting and protection circuit as recited in claim 5further including sensing means coupled to said activating means and tosaid output point for maintaining said activating means operative whensaid output voltage exceeds a predetermined level.
 7. A starting andprotection circuit as recited in claim 6 further including startingmeans coupled to said activating means for rendering said activatingmeans operative for a predetermined time interval.
 8. A starting andprotection circuit as recited in claim 7 further including OR circuitmeans having inputs connected to said starting means and said sensingmeans, and an output connected to said activating means, said OR circuitmeans being responsive to said starting means for rendering saidactivating means operative for a predetermined time interval, and tosaid sensing means for maintaining said activating means operative aftersaid predetermined time interval when said output voltage exceeds apredetermined level.
 9. A starting and protection circuit as recited inclaim 8 wherein said starting means includes means for repetitivelyrendering said activating means operative.
 10. A starting and protectioncircuit as recited in claim 8 further including control means connectedto said activating means and to said output point, said control meansbeing responsive to the voltage at said output point for increasing thetime during each period that said switch is maintained conductive inresponse to a decrease in the output voltage and for decreasing the timeduring each period that said switch is maintained conductive in responseto an increase in the output voltage.